Japanese Patent Application Laid-open Publication H6-310101 discloses conventional technology in this field. In a gas (deuterium) discharge tube described in this publication, two metallic partition walls are disposed on a discharge path between an anode and a cathode, a small hole is formed in each partition wall, and the discharge path is narrowed by these small holes. As a result, light of a high luminance can be obtained by means of the small holes on the discharge path. If three or more metallic partition walls are provided, even higher luminance is obtained, and the luminance of the light increases as the small holes are made smaller.